The present invention relates to dispersions of particles of photocatalytic titanium dioxide which are capable of being used to treat substrates.
It is known that titanium dioxide makes possible, by its photocatalytic activity, the degradation of organic or biorganic molecules.
If this photocatalytic titanium dioxide is deposited on a support, the surface of this support becomes oxidizing and dirty marks, in particular of organic origin, which are deposited there are destroyed by photooxidation. The surface is said to be self-cleaning.
The deposition of titanium dioxide at the surface of the substrate can be carried out from dispersions of titanium dioxide particles. Use is preferably made of dispersions of particles exhibiting a small size, in particular a nanoparticle size, so as to obtain translucent surfaces, in contrast to micrometric titanium dioxide, which gives white surfaces.
The treated surfaces can be glass, plastics, building materials (mortars, concretes, terracottas), ceramics, stones, paper or wood.
The deposit of titanium dioxide on these supports must adhere strongly to the support for the treated surfaces to be able to be installed and in order for them to retain their self-cleaning properties over time. It is also necessary for the binder which allows the particles to adhere to the support not to be sensitive to the photocatalysis of the titanium dioxide particles.
With this aim, several processes have been employed which provide various types of binders which allow the particles to be adhesively bonded to the substrate.
A first process consists in depositing dispersions of titanium dioxide particles comprising the precursor of a binder on the substrate under hot conditions. For example, provision has been made to use dispersions of titanium dioxide particles and of organometallic binders of titanate or silicate type. The particles are then held in a film of silica or titanium dioxide (this principle is disclosed, for example, in WO 97/10185). This inorganic binder exhibits the advantage of not being photodegradable.
A second process consists in depositing dispersions of titanium dioxide particles comprising an organic binder on the substrate under cold conditions. One problem is that this binder must not degrade under the effect of the photocatalytic properties of the titanium dioxide particles. To achieve this, provision has been made, for example, to choose the binder from silicones.
However, although the silicone binders provided do not degrade on contact with the photocatalytic particles, it is observed that they do not always result in a homogeneous, hard and adherent coating: very often, the coatings obtained can be removed by simple rubbing with the finger.
One aim of the present invention is therefore to provide dispersions of titanium dioxide particles and of a non-photodegradable binder which can be used to form photocatalytic coatings at the substrate surface under cold conditions.
Another aim of the present invention is to provide such dispersions, the use of which results in homogeneous, hard and adherent coatings.
With these aims, the invention relates to a dispersion of particles of photocatalytic titanium dioxide, in which dispersion the liquid phase comprises at least one crosslinking catalyst and at least one polyorganosiloxane, either of formula (I):
Mxcex1Dxcex2Qxcex4(O1/2Ri)xcex5,
or of formula (II):
Mxcex1Dxcex2Txcex3(O1/2Ri)xcex5,
as defined hereinbelow.
With these aims, the invention also relates to the use of this dispersion to treat substrates.
The dispersions according to the invention exhibit the advantage of being chemically neutral and of not interacting with the substrates on which they are deposited.
They also exhibit the advantage of employing inexpensive binders.
In addition, they make it possible, under certain conditions of use, to result in transparent or translucent coatings.
The invention relates first of all to a dispersion of particles of photocatalytic titanium dioxide, in which dispersion the liquid phase comprises at least one polyorganosiloxane:
either of mean formula (I):
Mxcex1Dxcex2Qxcex4(O1/2R1)xcex5,
xe2x80x83in which:
M=Rii3SiO1/2 
D=Rii2SiO2/2 
Q=SiO4/2 
with Rii, which are identical or different, representing either a linear or branched alkyl radical having from 1 to 8 carbon atoms or a substituted or unsubstituted aryl group having from 6 to 12 carbon atoms or an aralkyl, alkaryl, aryloxyalkyl or alkoxyaryl group in which the aryl group comprises from 6 to 12 carbon atoms, which atoms can optionally be substituted by at least one linear or branched alkyl or alkoxy group having from 1 to 4 carbon atoms, and in which the alkyl or alkoxy group has from 1 to 4 carbon atoms and is linear or branched,
xcex1, xcex2 and xcex4 respectively represent the molar fractions of the silicon atoms of the M, D and Q units, with xcex1+xcex2+xcex4=1, and:
xcex1xe2x89xa60.10, preferably xcex1xe2x89xa60.010,
xcex2xe2x89xa60.85,
xcex4xe2x89xa70.10,
Ri, which are identical or different, represent an alkyl group having from 1 to 4 carbon atoms,
xcex5 represents the mean number of O1/2 Ri units per silicon atom,
or of mean formula (II):
Mxcex1Dxcex2Txcex3 (O1/2 Ri)xcex5,
xe2x80x83in which:
M, D, Ri and xcex5 have the above meanings and
T=RiiSiO3/2, with Rii having the same meaning as above,
xcex1, xcex2 and xcex3 respectively represent the molar fractions of the silicon atoms of the M, D and T units, with xcex1+xcex2+xcex3=1, and:
xcex1xe2x89xa60.20, preferably xcex1xe2x89xa60.010,
xcex2xe2x89xa60.60,
xcex3xe2x89xa70.30.
Preferably, the polyorganosiloxane of the dispersion has the formula (I) or (II) and Ri is an ethyl or methyl group. On average, the polyorganosiloxane can also exhibit silanol ends (Ri=H), the said ends not representing more than 20% of all the ends.
According to a first preferred alternative form, the dispersion according to the invention comprises a polyorganosiloxane of formula (II) in which:
for each unit T=RiiSiO3/2, Rii is a methyl,
for each unit D=Rii2SiO2/2, one Rii substituent is a methyl and the other Rii substituent is an octyl,
xcex2 has a value of at most 0.10,
xcex3 has a value of at least 0.70.
According to a second preferred alternative form, the dispersion according to the invention comprises a polyorganosiloxane of formula (II) in which:
for each unit T=RiiSiO3/2, Rii is a methyl,
for each unit D=Rii2SiO2/2, the two Rii substituents are methyls,
xcex2 has a value of at most 0.30,
xcex3 has a value of at least 0.70.
According to a third preferred alternative form, the dispersion according to the invention comprises a polyorganosiloxane of formula (II) in which:
for each unit T=RiiSiO3/2 , Rii is a propyl,
for each unit D=Rii2SiO2/2 , the two Rii substituents are methyls,
xcex2 has a value of at most 0.40,
xcex3 has a value of at least 0.40.
The dispersions according to the invention can also comprise a crosslinking catalyst. The latter can be chosen from organic titanium compounds (for example, alkyl titanates) or organic tin compounds (for example, dialkyltin dicarboxylate).
Alkyl titanates are preferred.
The use of this catalyst is recommended for the use of the dispersion during the treatment of glass substrates.
The liquid phase of the dispersion according to the invention may comprise only a polyorganosiloxane as defined above or else may also comprise a solvent.
The solvent of the liquid phase of the dispersion according to the invention can be aqueous or organic.
It is generally an organic solvent. The solvent can be chosen from the solvents for the silicone polymers used, such as, for example, D4 (octamethylcyclotetrasiloxane) or other volatile siloxanes, white spirit, C1-C8 alcohols, or aliphatic or aromatic hydrocarbons, such as cyclohexane or alkanes.
The choice of the solvent is made according to its compatibility with the polyorganosiloxane. It is thus possible to vary the transparency of the final coating.
The dispersion according to the invention preferably comprises a solvent, in particular when it is desired to decrease the concentration of titanium dioxide in the coating in order to obtain coatings of greater translucency or greater transparency.
The photocatalytic particles of the dispersion according to the invention are preferably titanium dioxide particles exhibiting a size of at most 100 nm, in particular of between 10 and 50 nm. The diameters are measured by transmission electron microscopy (TEM).
The nature of the crystalline phase is, preferably, predominantly the anatase crystalline form. xe2x80x9cPredominantlyxe2x80x9d means that the level of anatase in the titanium dioxide particles is greater than 50% by mass. The particles preferably exhibit a level of anatase of greater than 80%.
The degree of crystallization and the nature of the crystalline phase are measured by X-ray diffraction.
It is preferable to use monodisperse titanium dioxide particles in order to obtain coatings of greater transparency. Monodisperse is understood to mean particles exhibiting a dispersion index of at most 0.5, preferably of at most 0.3, the dispersion index being given by the following formula:   I  =                    φ        84            -              φ        16                    2      ⁢              φ        50            
in which:
Ø84 is the diameter of the particles for which 84% by weight of the particles have a diameter of less than Ø84,
Ø16 is the diameter of the particles for which 16% by weight of the particles have a diameter of less than Ø16,
Ø50 is the mean diameter of the particles.
The diameters of use in the determination of the dispersion index are measured by centrifugal sedimentation of the particles of the dispersion, monitored by X-rays, using a Brookhaven-type XDC device.
The monodisperse particles of the dispersion preferably result from a so-called solution or wet-route preparation process (thermolysis, thermal hydrolysis or precipitation of a titanium salt), in contrast to processes for the high-temperature pyrolysis or oxidation of a titanium salt. They can be, for example, titanium dioxide particles obtained by the process described in Application EP-A-0,335,773.
It can in particular be the preparation process which consists in hydrolysing at least one titanium compound A in the presence of at least one compound B chosen from:
(i) acids which exhibit:
either a carboxyl group and at least two hydroxyl and/or amine groups,
or at least two carboxyl groups and at least one hydroxyl and/or amine group,
(ii) organic phosphoric acids of following formulae: 
in which n and m are integers of between 1 and 6 and p is an integer of between 0 and 5, R1, R2 and R3, which are identical or different, representing a hydroxyl, amino, aralkyl, aryl or alkyl group or hydrogen,
(iii) compounds capable of releasing sulphate ions in acidic medium,
(iv) salts of the acids described above, and in the presence of anatase titanium dioxide seeds exhibiting a size of at most 5 nm and in a ratio by weight of TiO2 present in the seeds/titanium present before introduction of the seeds in the hydrolysis medium, expressed as TiO2, of between 0.01% and 3%.
This process for the preparation of the particles thus comprises several stages and, firstly, a stage of preparation of the starting solution comprising a titanium compound A, a compound B as defined above and titanium dioxide seeds.
This starting solution, intended to be hydrolysed, is preferably completely aqueous; another solvent, for example an alcohol, can optionally be added, provided that the titanium compound A and the compound B used are then substantially soluble in this mixture.
As regards the titanium compound A, use is generally made of a compound chosen from halides, oxyhalides or alkoxides of titanium, sulphates and more particularly synthetic sulphates.
Synthetic sulphates is understood to mean titanyl sulphate solutions produced by ion exchange from very pure titanium chloride solutions or by reaction of sulphuric acid with a titanium alkoxide.
The preparation is preferably carried out with titanium compounds of the titanium halide or oxyhalide type. The titanium halides or oxyhalides which are more particularly used in the present invention are titanium fluorides, chlorides, bromides and iodides (respectively oxyfluorides, oxychlorides, oxybromides-and oxyiodides).
According to a particularly preferred form, the titanium compound is titanium oxychloride TiOCl2.
The amount of titanium compound A present in the solution to be hydrolysed is not critical.
The initial solution additionally comprises at least one compound B as defined above. Mention may be made, as non-limiting examples of compounds B coming within the scope of the present invention, of in particular:
hydroxypolycarboxylic acids and more particularly hydroxydi- or hydroxytricarboxylic acids, such as citric acid, maleic acid and tartaric acid,
(polyhydroxy)monocarboxylic acids, such as, for example, glucoheptonic acid and gluconic acid,
poly(hydroxycarboxylic) acids, such as, for example, tartaric acid,
dicarboxylic amino acids and their corresponding amides, such as, for example, aspartic acid, asparagine and glutamic acid,
hydroxylated or non-hydroxylated monocarboxylic amino acids, such as, for example, lysine, serine and threonine,
aminotri(methylenephosphonate), ethylenediaminotetra(methylenephosphonate), triethylenetetraaminohexa(methylenephosphonate), tetraethylenepentaaminohepta(methylenephosphonate) or pentaethylenehexaaminoocta(methylenephosphonate),
methylenediphosphonate, 1,1-ethylenediphosphonate, 1,2-ethylenediphosphonate, 1,1-propylenediphosphonate, 1,3-propylenediphosphonate, 1,6-hexamethylenediphosphonate, 2,4-dihydroxypentamethylene-2,4-diphosphonate, 2,5-dihydroxyhexamethylene-2,5-diphosphonate, 2,3-dihydroxybutylene-2,3-diphosphonate, 1-hydroxybenzyl-1,1-diphosphonate, 1-aminoethylene-1,1-diphosphonate, hydroxymethylenediphosphonate, 1-hydroxyethylene-1,1-diphosphonate, 1-hydroxy-propylene-1,1-diphosphonate, 1-hydroxybutylene-1,1-diphosphonate or 1-hydroxyhexamethylene-1,1-diphosphonate.
As already indicated, it is also possible to use, as compound B, all the salts of the abovementioned acids. In particular, these salts are either alkali metal salts, more particularly sodium salts, or ammonium salts.
These compounds can also be chosen from sulphuric acid and ammonium or potassium sulphates.
The compounds B as defined above are preferably hydrocarbonaceous compounds of aliphatic type. In this case, the length of the main hydrocarbonaceous chain preferably does not exceed 15 carbon atoms and more preferably 10 carbon atoms.
The amount of compound B is not critical. The molar concentration of the compound B with respect to that of the titanium compound A is generally between 0.2 and 10% and preferably between 1 and 5%.
Finally, the starting solution comprises titanium dioxide seeds used in a specific way.
First of all, the titanium dioxide seeds used in the present invention must exhibit a size of at most 5 nm, measured by X-ray diffraction. Use is preferably made of titanium dioxide seeds exhibiting a size of between 3 and 5 nm.
Subsequently, the ratio by weight of the titanium dioxide present in the seeds to the titanium present in the hydrolysis medium before introduction of the seeds (that is to say contributed by the titanium compound A) and expressed as TiO2 is between 0.01 and 3%. This ratio can preferably be between 0.05 and 1.5%. The bringing together of these two conditions with respect to the seeds (size and ratio by weight), in combination with the process as described above, makes it possible to precisely control the final size of the titanium dioxide particles, a level of seeds being associated with a particle size. It is thus possible to obtain particles for which the size varies between 5 and 100 nm.
Use is made of titanium dioxide seeds in the anatase form, so as to induce precipitation of the titanium dioxide in the anatase form. Generally, due to their small size, these seeds instead exist in the form of poorly crystallized anatase. The seeds are generally provided in the form of an aqueous suspension composed of titanium dioxide. They can be obtained in a known way by a process of neutralization of a titanium salt by a base.
The following stage consists in hydrolysing this starting solution by any means known to a person skilled in the art and generally by heating. In the latter case, the hydrolysis can preferably be carried out at a temperature greater than or equal to 70xc2x0 C. It is also possible to operate, firstly, at a temperature below the boiling temperature of the medium and, then, to maintain the hydrolysis medium level at the boiling temperature.
Once hydrolysis has been carried out, the titanium dioxide particles obtained are recovered by separation of the precipitated solid from the mother liquors. They are then redispersed in an aqueous liquid medium so as to obtain a titanium dioxide dispersion. This liquid medium can be acidic or basic.
It has been observed that the titanium dioxide particles resulting from a so-called solution or wet-route preparation process, and in particular resulting from the process described above with hydrolysis at a temperature of approximately 100xc2x0 C., exhibit, because of their porosity, a lower refractive index than the titanium dioxide particles resulting from other processes. This property is of great interest when the particles are used to prepare a coating on a glass substrate, because the coating obtained also exhibits a low refractive index. This optical advantage is important because a layer of titanium dioxide with a high index results in an increase in the light reflection of the carrier glass and thus in a decrease in its light transmission. In point of fact, in certain applications, in particular in the field of glazings for installation in vehicles, it is essential to have high light-transmission levels (for a windscreen, a minimum light transmission of 75% is necessary).
The particles of the dispersion preferably exhibit a BET specific surface of at least 70 m2/g.
BET specific surface is understood to mean the specific surface determined by nitrogen adsorption in accordance with ASTM Standard D 3663-78, drawn up from the Brunauer-Emmett-Teller method described in the periodical xe2x80x9cThe Journal of the American Chemical Societyxe2x80x9d, 60, 309 (1938). In order to measure the specific surface of the particles according to the invention when they are provided in the form of a dispersion, it is essential to follow the measuring protocol, which consists in removing the liquid phase from the dispersion and in then drying the particles under vacuum at a temperature of 150xc2x0 C. for at least 4 hours.
The particles of the dispersion preferably also exhibit a relative density of the order of 2.4. xe2x80x9cOf the orderxe2x80x9d is understood to mean that the relative density is 2.4xc2x10.2. Such a relative density value is low with respect to the conventional relative density of anatase titanium dioxide, which is 3.8. This relative density is evaluated by measurement of the pore volumes.
These specific surface and relative density features can be obtained for the titanium dioxide particles resulting from a so-called solution or wet-route preparation process and in particular resulting from the process described above with hydrolysis at a temperature of approximately 100xc2x0 C.
The proportions of the particles to the polyorganosiloxane of the dispersion according to the invention vary with the use which is made thereof. Thus, for the application of the dispersion as a coating on concrete, the proportion of the particles generally represents at least 5% by weight of the particles+polyorganosiloxane mixture; on the other hand, for the application of the dispersion as a coating on glass, the proportion of the particles generally represents at least 10% by weight of the particles+polyorganosiloxane+optional crosslinking catalyst mixture, preferably at least 50% and generally at most 90%.
The solvent of the dispersion according to the invention is generally present in an amount such that the solids content, as titanium dioxide particles, is at least 0.5% by weight.
If a crosslinking catalyst is present in the dispersion, it generally represents at least 5% by weight of the polyorganosiloxane+catalyst mixture and preferably at most 50% by weight.
The invention also relates to the process for the preparation of the above dispersions, which consists in simply mixing the titanium dioxide particles and the polyorganosiloxane.
The polyorganosiloxanes which can be employed in the dispersion are known and commercially available.
The titanium dioxide particles can also be obtained commercially. They can be provided in various forms.
They may first of all be aqueous dispersions of titanium dioxide particles, such as those sold by Rhxc3x4ne-Poulenc under the name S5-300, or dispersions obtained according to the process disclosed in Patent EP-A-0 335 773, as described above.
Use is preferably made of basic aqueous dispersions, as it has been observed that the latter result in dispersions which give coatings of greater transparency than acidic aqueous dispersions. In the case of deposition on glass, this difference can be reduced if the glass is activated with NaOH before deposition.
They may also be organic dispersions of titanium dioxide particles. These can be prepared from aqueous dispersions of titanium dioxide particles, the phase transfer being carried out, for example, according to one of the following methods:
washing with acetone or the desired solvent by centrifuging and redispersing in the organic solvent,
azeotropic distillation of the water/solvent mixture, if the water and the solvent are immiscible and form an azeotrope,
evaporation of the water on a rotary evaporator, if the solvent is miscible with water and boils at a temperature greater than that of water,
mixing an aqueous dispersion with an organic medium comprising a cationic transfer agent, if the particles are negatively charged, it being possible for the transfer agent to be chosen in particular from quaternary amines or quaternary ammonium salts, or a medium comprising an anionic transfer agent, if the particles are positively charged (this process is described more particularly in Patent GB-A-988,330).
Use may also be made of titanium dioxide powders. Such powders are commercially available; mention may be made of the G5 or DT51D powders sold by Rhodia Chimie. Powders can also be obtained by atomization of an aqueous dispersion as described above.
The dispersions according to the invention are preferably prepared from particles in the form of dispersions, in particular when it is desired to obtain a transparent surface treatment, powders generally resulting in coatings of reduced transparency.
The invention also relates to the use of the above dispersions in treating the surface of a substrate.
The polyorganosiloxane acts as binder in order to attach the particles to the substrate.
The substrate can be of various kinds: it may be, for example, glass, polymers (plastics), building materials, such as mortars, concretes or terracottas, ceramics, stones, wood, metals or paper.
In the case of the treatment of alkaline substrates and in particular of concrete, use is preferably made of the dispersion defined in the third alternative form hereinabove, namely that comprising at least one polyorganosiloxane of formula (II) in which:
for each unit T=RiiSiO3/2, Rii is a propyl,
for each unit D=Rii2SiO2/2, the two Rii substituents are methyls,
xcex2 has a value of at most 0.40,
xcex3 has a value of at least 0.40.
If the polyorganosiloxanes exhibit intrinsic properties, such as, for example, protection of the substrates (water repellency and the like), it has been observed that the fact of mixing them with the titanium dioxide particles does not modify their properties. This is the case, for example, with the coating employing polyorganosiloxane of the third preferred alternative form, which makes it possible to obtain a coating which adheres firmly to the alkaline substrate and, furthermore, provides a water repellency property specific to this type of polyorganosiloxane binder.
Deposition can be carried out by any conventional method: roller, brush, spray gun, spray.
The following examples illustrate the invention without, however, limiting the scope thereof.